Method of and apparatus for seismograph prospecting



Oct. 21, l941. E. A. ECKHARDT EI'AL 2,260,217

- METHOD OF AND APPARATUS FOR SEISMOGRAPH PROSPECTING Filed March 14, 1939 5 Sheets-Sheet 1 RECORDER 72 AMPLIFIERS 3M W a Oct. 21 1 METHOD OF .AND APPARATUS FOR SEISMOGRA'PH PROSPECTING E. A. ECKHARDT EI'AL 2,260,217 I Filed March 14, 1939 5 Sheets-Sheet 2 TIME 13-210 HY ON on on REISPONSIVENES D //LV DIRECT 32555 =RF1CTD EVEN-s ARRIVAL RECORD I? DETECTORS Oct. 21, 1941. E. A. ECKHARDT ETAL 2,260,217

METHOD OF AND APPARATUS FOR SEISMOGRAPH PROSPECTING Filed March 14, 1939 5 Sheets-Sheet 3 7.z5 7 z 1 r? 72.9

DETECTOR MP KIA 1 DET 75 735 272 729 BET.

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1 -1 n-no DETECTOR M10 RESPONSIVENESS D'51O ransom 1m DA 3L3; gggzg g REFLECTIONS gmcmms skm W5 E. A. ECKHARDT ETAL vMETHOD OF AND APPARATUS FOR SEISMOGRAPH PROSPECTING Filed March 14, 1939 SSheets-Sheet 5 BLIFSSTING MACHINE R MW/Ly Patented Oct. 21, 1941 METHOD OF AND APPARATUS FOR SEISMO-.

GRAPH PROSPECTING Engelhardt A. Eckhardt, Pittsburgh, and Gary Mumy, Penn Township, Allegheny County, Pa., assignors to Guli Research & Development Company, Pittsburgh, Pa,

Delaware a corporation oi.

Application March 14. ms, sci-m 281,854

(on. iii-0.5)

Claims.

This invention or discovery relates to methads at and apparatus ior seismograph prospecting; and it comprises a method at seismograph prospecting including the steps oi establishing a plurality -.oi' seismic wave detectors at spaced points-in the earth, establishing a recording element therefor, generating a seismic wave impulse at a point spaced from the detectors, transmitting signals irom one detector only to said recording element at the time oi arrival oi the flrst wave at said .detector and transmitting signals from a pluraiity'oi detectors to said recording element at the time oi arrival oi= reflected waves at said detectors; and it further comprises apparatus adapted ior carry ing out such method and including means ior generating-seismic waves in the earth, a group of detectors ior detecting seismic waves, amplitying-and recording means, andswitching means between the detectors and the recording means constructed and arranged to connect'one oi the sled oi a seismograph prospecting system including' a group oi detectors and an electronic circuit for each detector, wherein the flrst wave arriving at a detector is received and recorded by itself, the other detector circuits remaining out'oi'action, and then all the detector circuits are rendered sensitiveior receipt and recording point in all directions.

. trating downward any great distance.

detectors to the recording means iorunobstructed receipt oi the first reiracted wave to arrive at said detector and to connect a plurality at detectors to the recorder means for receipt oi reflected waves; all as more fully hereinaiter set forth and as claimed.

One object of the invention is theprovision oi a method oi reflection seismograph prospecting with the aid of a plurality oi detectors and a. single recording element, in which a single detector isconnected 'to the recorder for reception oi the refracted waveand a plurality ci detecbrs areconnected to the recorder ior recaption 'oi the reflected waves, whereby a clear record oi both types at waves is obtained in a simple manner. 1

qnother'object of the invention is the provision oi a seismosraph prospecting system influding methods oi and means for systematical- 1y recording the signals irom a plurality .oi seismic detectors in such sequence as to yield in the, record the most iniormation from a given number-oi detectors. Another object is the provision oi a remote controlled switching system for; detectors for systematically connecting the detectors to the recorder during the receipt 0! seismic. waves in such a way as to receive both the direct arrivals, and later arrivals, separately and without mutual obstruction. Another obiect is theprovision oi a switching system in' which the detectors are automatically connected .to or disconnected irom a recorder in a systematic manner, up n arrival oi some particular wave at each detector. Another object is the proviorder to make corrections ior the characteristics oi later arriving waves.

In seismograph prcspecting,' an explosive charge is set oflat some point in the earth, and

seismic waves are propagated irom this shot Seine waves travel along near' the surface of 'the earth without pene- Other waves are'reflected and reiractedat interfaces between deep buried rock strata oi different wave-transmitting'characteristics and returnto the surface oi the earth in the general neighborhood oi the shot point. In the reflection method oi seismograph prospecting; a plurality oi seismic detectors or seismophones are placed in theearth at spaced distances, so located with respect to the shot point as to receive waves reflected upward irom deep strata. The detectors are arranged to give a fluctuating electrical sig-- ml which corresponds to the amplitude oi the earth tremors at the detector, or to the .velocit or the acceleration oi the tremors, depending upon the type oi seismophone employed. The electrical signal is ampli'iled and is recorded for study'. The record is made by applying the amplifl'ed detector signals to oscillograph elements (sensitive galvanometers capable of registering rapid vibrations) arranged to produce a trace on a moving bandoi photographic paper or In reflection seismograph. work, it is also necessary to record andstudy the so-called direct waves (shallow reiracted waves which travel more or less directly from the shot point to the detectors near the surface oi th'e earth), in

oi the weathered layer' oi rock Just below the earth's surface. This weathered layer, which may be oi thickness irom zero to many ie'et. has wave-transmitting characteristics markedly dii-,

ierent from those oi the underlying more compact rock, and its eiiect on waves reflected up irom deep strata must be measured and compensated iori flrst to arrive at the detector positions.

In good current reflection seismograph practice, a'plurality-oi series-connected, spaced detectors are employed ior each oi the recorder oscfllolraph elements, instead oi a single de- The direct wave is ordinarily the typical I apparatus embodiment of 2 tector for each recorder element as in older practice. Several.such groups are employed. The resultant output of each group of detectors. is

recorded as a singletrace which is considered as if it were due to ahypothetical single detector centrally located in the group. This arrangement. is particularly advantageous in studying the reflections from the deeper underlying strata, because it results in a clearer record of such reflections.

However, the detector grouping arrangements described are not specially satisfactory for recording the direct wave. They are no better than the first arrival. In fact, the direct wave, which is usually much more intense than any of the reflected waves seriously obscures the record.

According to the invention, there is provided a system wherein grouped detectors are em- I ployed, but only one detector 'at a time is allowed to recordthe direct wave the other detectors of the group being connected to the recorder only after the direct wave has been received and recorded. This gives the necessary data for making the weathering correction andat the same time the record for both the refracted wave and the reflected waves is much improved. In one form of the invention, the first arrivals at a plurality of detectors are recorded separately, each detector in turn being momentarily connected to the recorder during receipt of the direct wave at -the detector, then disconnected so that the signal from the next seismophone will not be obscured, and finally connected again to the recorder in time for receipt of. the several reflected waves. Such arrangement gives the greatest amount, of data possible, with a given number of'detectors. The invention'is also susceptible of other applications, as will be clear from the extended de-v scription to. follow.

In the accompanying drawings-there are shown diagrammatically four examples of specific embodiments of apparatus within the purview of the invention and useful in the perfcrmanceof the method, and diagrams illustrative of the actions taking'place. Inthe drawings- Fig. l is a'view of the complete apparatus organization set up in the field; v

Fig. 2 is adetailed diagram of one form of apparatus including mechanical switching means; Fig. 8 is-a chart illustrating the detector. switching as' obtainedwith the apparatus of.

63 and a motor '64 for moving the paper.

Fig. 4 is a diagram of a modification of Fig.

2,. in which the switching mechanism is' under automatic electrical control;

in a simple form as including detectors arranged in two groups of three (ll, llll and 2M, l0, Ill! and 210') switching apparatus II and II and amplifiers l2 and I2, one for each group, and a recorder IS. The switching apparatus is located near the detectors and connection is made to the amplifiers by cables I98 and I98 (see also Fig. 9). The detectors are set up at 'ornear the surface of the earth, and receive waves from ya shot ll fired by a blasting machine It. The

- shot is usually located at the base of the weathered layer of rock I6 near the surface of the earth, Waves are propagated from the shot in all directions. as indicated by rays in the figure.

the inter-- A so-called direct wave travels along face I! between the weathered layer and a lower, more consolidated layer It, and reaches the several dectors by bent paths as indicated at I91 The direct wave usually reaches the detector first. Other waves, indicated by rays III, penetrateinto the earth and reach the detectors after reflection from deep strata interfaces II and i2.

\ Generally these reflected waves arrive at the detectors later than the direct wave.

One form of switching apparatus ll, arranged for utilization with three detectors, is shown in Fig. 2. The apparatus includes the elements surrounded by the dot-dash line as shown, and is conveniently placed at or near the detectors as indicated in Fig. l; at a location which may be several hundred or thousand feet away from'the amplifying and recording system. The apparatus is designed to permit connection of any one of the three detectors to the recorder, and to permit connection of allthree detectors simultaneously to the recorder, as desired. A four-point double-pole switch is provided, shown as including two stationary plates" and 20 of insulating material, each having four contact points 21, 28',

29, 30 and ll, 32, 31 and i4, cooperating with a pair of rotary brushes 3! and 36 mounted on a shaft 31. The brushes are insulated from each other by insulation 38. Each detector has a pair of leads 39 and ill, II and 42. and ll and 44 respectively, and each lead is connected totwo of the contact points. This lead I! is connected Al is connected to points 33 and 34 .by wires'il and U2. The brushes, 3! and 30, are connected to' the amplifier I! through leads II, a transformer 84, leads l5 and a transformer 58 as.

shown. The amplifier is of conventional electronic typeand delivers an' amplified signal through leads Slto the recorder. l8, which also is of known type and-includes an oscillograph element ill provided with a mirror (not shown), a lamp '2, a moving photographic paper surface The oscillograph element on actuation causes a focused beam of light from the 'lamp to move over Fig. 5 is 'a diagram of an electronic switching y em;

Fig. 6 is a'chart illustrating the actions taking, place in the utilization of the apparatus of Fig. 5;

Fig. 7 is a diagram of a modified form of the apparatus of Fig. .5;

- Fig. 8 is a diagram of a' modification of the showing ,of the system 1 the photographic paper and make a trace .65. The blasting machine II is connected to the shot I (a charge of dynamite with an electric blasting cap 86) through leads .1 and an oscillograph eiementil is connected into this shot-firing circuit,-through leads as with a large protective shuntresistance '8' arranged as shown, so that on firingthe shot a fluctuation will be produced on the paper by oscillograph 6| which will allow the instant'oflfiring the shot to be determined on the record. 7

II. II, 24 .does not interfere'with of the detector-amplifier circuit.

- hand detector I is connected to the amplifier.

I are as follows: Shaft 11 is urged in a clockwise direction by a torsion spring II, the brushes normally resting against stops Ii on plates 2! and 20. A polarized relay is provided including a triple pole piece 11, a T-shaped permanently magnetized armature ll arranged for rotationabout a pivot II and normally held in the neutral position shown by a pair of springs 20 and II. and having a pawl I2 engaging a ratchet-l2 on shaft TI. Pole piece 11 is energized by a coil 84 which is connected through a reversing switch I! with a battery It, by meansof lead 81 (from I the coil to a center tap in transformer ll) lead II (to one side of the reversing switch) and lead II (from the other side of the reversing switch to a center tap on transformer SI) Upon energization of coil 04, both the upper and the lower poles of pole piece 11 are energized, with'like polarity, N or 8 depending on the position of switch 25. A cooperating detent mechanism is dred or thousand feet long; The arrangement of wires and I2 is in the form of a phantom circuit, whereby actuation of the m netic switch e functioning With reversing switch run one position, the upper end (N) of :the T-shaped member II is drawn toward the pole piece 11 and ratchet 03 is moved in an anti-clockwise direction. Detent Ill holds the ratchet wheel against reverse movement. By momentarily opening and closing the switch the proper number of times, any one of the detectors can be connected to the amplifier. With the reversing switch I! in its other position, the polarity of pole piece 11 is ncversed,

and by momentarily closing the switch to this position, detent arm llll releases the teeth of ratchet wheel '2, allowing the shaft 3L to rotate clockwise under the influence of sprin 15. In the position shown, all three detectors are connected to the amplifier in series; 83* actuating switch ll until brushes I! and I are caused to make contact at points 29 and 33, only the right- When brushes "and 36 contact points 2' and 32, onlythe central detector III is connected, and when brushes contact points 21 and SI. only the left-hand detector 2" is connected.

In-operation, assuming the right-hand detector It to be the detector nearest the shot point (cf. Fig. i) switch ll, will first be operated manually to bring brushes II and IS in contact with points 22 and II, so that only this first detector is connected with the recorder when the direct wave reaches it. When switch II is reversed momentarily the shaft 21 is brought back to .the position sliown in Fig. 2 under the influence of spring again to connect some other detector, for the next shot. When the position shown in Fig. 2 is attained, all detectors are connected to the amplifier in series. The switching device of Fig. 2 can thus be used as a remote-operated manual switch. It forms a very convenient way of shift- 76 The switch I! can then be manipulated ing the detector connections at will and saves the services of an extra operator which might be required were thedetector connections to be changed at the recordingpoint or were extra cables to be run. Y i

. When the time interval'between the first arrivals and the reflection arrivals is sufficiently provided, including a bell crank III with detent t H in the record as indicated in Fig. 3. but they are characteristic in form and easy to distinguish,

great, the switching mechanism of Fig. 2 can be operated between the time of the first arrival and the time of the reflection arrival, so as to record as .a single oscillograph trace the first arrival, and reflection, arrivals. from a single shot. The functioning of the system of Fig. 2 in such procedure is diagrammed in Fig. 3, which shows by hatching the "on" and "of!" intervals during" which the several detectors are connected to the amplifier and recorder. As shown, initially only detector I0 is connected, .so as to receive the direct-wave arrival (first arrival) under optimum conditions. Then the other two detectors iii! and 2l0 are connected, to' record the reflection arrivals. The relative times at which the direct wave and the reflected waves arrive are indicated switching from the first detector to all the de-' tectors'is made between the instants when the direct wave and the reflected waves arrive. The

switching usually introduces some false signals and moreover they arrive at a time when no important waves are being received; so there is no confusion from this source.

Fig. 4 shows a modification of the apparatus of Fig. 2, having two automatic controls for the multi-point switch, either of which can be employed according to the particular circumstances encountered. As shown, magnetic means are provided for actuating crank llll, comprising an electromagnet I" adapted to draw armature )2 so as to release the ratchet. The magnetis energized through "leads lli'l bythe being interposed asshown. Wires I01 are conveniently included in cable I" along with wires A it and II. With the arrangement shown, the

connected to the recorder.

The system is shown with an alternative control adapted to actuate the multi-point switch a predetermined, brief interval after the firing cases. As shown. .a bimetallic (thermostat) element 2 is provided, adapted tobe heated by a heating resistor I II in the blasting cap cir- 'cuit (leads .1), which bimetallic element opposes a pair of contact points Ill and II! and thereby makes up a delaying switch. The switch is in series. through a double-pole, double-throw switch III, with leads II and I and the battery 86, so that, assuming switch I" to be thrown to the left and shunt switch I ll opened, upon firing of the shot, fi.rst only one of the detectors is connected to the recorder, but as soon as switch 2 closes, the circuit through coil I is completed and switching takes .place. By throwing the switch I" to. the right, the multi-point switch oNhe shot, which is more convenient in some a third variable tap I, on resistor I II. ing I36 is connected to the grid I23 of tube 323, and to ground. ,Winding I33 is connected to 4 I can be returned to starting position. The time delay of thermal switch 2 can be adjusted by .means of an adjustable resistor- III shunted being applied to the grid I20 of a control tube,

2:, m, m and an. Each tube has cathode LII grounded at I22, and a plate I22. Four transformers are provided, I23, I23, I21 and I23, the secondaries I23 of which are of identical construction and are connected in series to recorder "I3. (The usual filters, extra stages of amplification, etc., can be interposed between the transformers and the recorder if desired).

Each transformer has an ordinaryinput winding together with anextra winding or windings to iurnish control voltages. Thus transformer I23 has a primary I39 and two auxiliary windings m and 132; transformer I23 has a primaryi33 and an auxiliary winding I34; transformer I21 has a primary I33 and an auxiliary winding I33; and transformer I2! has a primary I31. and an auxiliary winding I38. For the plate circuits, one side of each of the primaries I33, I33, I33 and I3l is connected,.through a battery 143, with ground at Ill, while the other side of each of the primaries I33, I33, I33 and I3! is connected to the plate I22 of the corresponding tube, 35 as shown. The otherelectrical connections m as follows: auxiliary windings I3I, I32 I33, I33, and I38 respectively have capacitances 2, I43, I, I4! and Ill, and resistances ll'l, 3,133, In andv ItI, shunted thereacross to form filters, and rectifiers I52, I33, I I3, I33 and I" in series as shown. One side of winding I3I is connected, through rectifier I32, 0. battery I33 and a re- 7 sistor IBI, with the grid of tube 23. The other side of winding I3I is connected by a variable tap I82 with resistor III.

Winding I32 is connected between the rid I23 of. tube- I23, and a variable tap I63 to resistor III. Winding I34 is connected between the grid I23 of tube 223 and the resistor "I and also to the cathode of tube 323. Batteries I3, I" and I", and resistors I63, I III and The mode of. operation of the system of Fig. 5 is as follows: Assuming the system to be set up, waiting for the shot to be fired, only the first tube, 23, is in operative state, the other tubes 6 being biased beyond cutofi and therefore inoperative. Then when the first wave from the snot point reaches detector Ill. thesignal from the detector is amplified by the corresponding tube 23 and the amplified signal is delivered the recorder. However, immediately on receipt of the signal. a potential appears at transformer controlwinding "I. This potential is rectified at I52, and filtered at the resistance-acapaci-' tance combination I41, I32, and is the-grid I29 of the tube (23) to biasit. Thus a short interval after receipt of the first arrivalat tube 23, the tube is rendered inoperative or incapable of transmitting signals by being biased beyond cutoff, so the detector is in effect cut of! 75 Four detectors, III, IIII, III and 3I3 voltage, which is rectified at I33 and filtered at wind-- applied to 70 from the recorder. The resistance of the rectifier I52 and resistor Ill and the capacitance of condenser I32 are selected so as to let tube 23 remain inoperative over the required short time interval. In the meantime, while tube 23- is amplifying signals a second control voltage ii furnished at transformer winding I32, and this voltage. rectified at I33 and filtered at I43 and I, is a plied to the grid of tube I23 in such way as to render the grid less negative. 1. e. to unbias' it, whereby tube I23 is rendered capable of amplifying the first arrivai'at detector Ill.

Tube I23 stays on" for abrief instant and then is out ch, after tube 23 goes out of action as described and the unbiasing voltage from winding I32. is no longer available. The values of the components of filter I43, I48 are selected to keep tube I23 on the proper length of time.-

Tube I23 in turn'cuts in tube 223. Tube 223 cuts in tube 323. The time constant of filter I. In is made much larger than that of the first two filters described, so that tube 323 stays on for several seconds to insure recording of the first arrival and allthe reflected events.

When tube 323 receives its first arrival from the detector 3III, winding I33 produces a-control I, IBI, and is applied to the first three tubes (voltage being taken to the grid circuits of these tubes from resistance III at taps I32, I33 and" I), which makes the other three tubes active again for reception of the reflected waves. Filin I, III has along time constant of discharge to keep the tubes sensitive until all reflections arrive; usually a matter of a few seconds. After all waves have been received, the voltages in the various filters gradually die out, restoring the system to its original, state.

Fig. 6 illustrates the functioning of the of Fig. 5. Tube.23 is operative in the When the direct arrival reaches detector I3, develops an electrical signal which is amplified by tube is and recorded as indicated at DA in Fig. 6. 'The tube then g es out of action and tube I23 goes'into action to receive the direct arrival DA at detector III. Tube I23 goes out of action and tube 223 goes into action to receive the direct arrival DA" at detector 2". .Tube 223 then goes out of action and tube 323 goes into action to receive direct arrival DA' at detector 3" and stays in operative condition. Tube323 operates to bring the first three tubes into action again so that all the tubes are in operative state when the reflections come in, as shown. In due III, are included in the circuits of 55 windings I32, I33 and I33, as shown.

course, the transient'control voltages die out, restoring the system to its initial state as indicated at the extreme right of Fig. 6.

While the signals are shown as applied to a single recorder, separate recorders can of course be used if desired; a separate recorder being connected to the output winding I23 of each of the transformers I23, I23, I21 and I23. The elec- .tronic switching means of Fig. 5 is conveniently placed adjacent the amplifying and recording means. as shown.

i The control voltage appearing across the transformer winding I3I is, 'as described, used to bias the grid of tube 23 so as to render the tube inoperative. matic volume control, except that the amplifiers 23, I 23, 223, and 323 are more abruptly and completely cut oif. or. on than with usual automaticvolume-controlied amplifiers. It is sometimes advantageous to provide additional bias control tothe tubes in amplifiers I2, 2, 2I2, and 3I2,

This is similar in effect 'to autoas the control eii'ect can be improved by increasing-the number of tubes to which bias is applied. Figure "I is a partial view showing a modification of Figure 5, in which instead of a simple ampli- -fier I! there isused an amplifier 4i! arranged for. volume control in accordance with an applied grid bias voltage. Amplifier'lll, which is oiia general type known per se in radio circuits and which requires no detailed description, includes a preamplifier tube 3 and an amplifier tube Ill. Grids ill and ll. of these tubes are arranged to receive bias voltage through lead ill from rectifier III and transformer winding Ill.

age in transformer winding I 3| (as described in connection with Figure 5) a part of the control energy serves to bias the grid III of tube It to secure the desired throttling action and the remainder of the control energy to bias grids lll and III ,of amplifier II! and thereby bring about additional throttling eil'ect. By causing the controlvoltagetoaflectthreetubesinthemanner described, the controlling action is sharper than when only one tube is aifected. Figure 7 shows only one. detector-amplifier-control-tube circuit:

. 5 shafttltosprlngbacktoits original position, vis..thepositionshowninl"ig.8.whereall-the. detectorsareconneciedtotheamplifler for recelpt of the reflected waves. 11 the reflections arrivesimultaneously, the selector switch would then move. one step counter-clockwise and the system is ready for a new operation.

Fig. 9 is a diagrammatic showing oi the system of the-present invention as set up in the field, basedonFig.l. Whilethedrawingisnotto scale, it shows in a clear manner the problem In operation, on development oi a control voltthe other amplifiers-Ill, ill, and SI! of Figure 5 can bemodified exactly as shown in Figure 7.

Fig. 8 shows a modification of the apparatus of Fig. 2 which is adapted toshift the detector connections so that the detectors receive the direct wave successively and individually. and all the detectors receive the reflected waves. The

encountered in the selsmograph'prospecting art, and the solution of it according to the present invention. The shot II is ordinarily at a distance of from several hundred yards to several miles from the nearest of the detectors II. III. etc.,

and the complete array of detectors (six m shown) extends over a distance of from-several hundred to several thousand yards. The ampliflers l2 and recorder l8 are mounted in a truck Ill near the detectors, and the truck will or" dinarily be several hundred or thousand yards from the farthest detector (detector II in the figure). If each detectorwere connected independently to the amplifier, there would have" to be twelve wires. But according to the invention thconlywiresthathavetobelaidgreatdistanoes system of Fig. 8 thus performs a function similar to that of the apparatus of Fig. 5. Referring to Fig. 8, three detectors II, III! and III are shown as in Pig. 2: and multipoint switches and ll,

electromagnetic ratchet mechanism ll, I8, 4,,

amplifierll,recorder ilandshotllareallprovided as described in connection with Fig. 2. A third multipoint switch, having a stationary plate I" with contacts Ill, "2,1", I and I" is provided. Contacts It! to "Lara dummies. Shaft I1 is prolonged as shown-and carries a rotary brush or anfn I" enga ing the con-' tacts as shown. A pawl Ill and actuating electromagnet III are provided as in Fig. 4. The

along the ground are the two cables I and Ill. Thus the Job of laying cables isjust as simple in the present invention as in conventional prospecting systems without. any switching provisions. -Wires from the blasting machineas shown at 0| (or .a corresponding radio transmit ting circuit) are provided in the present system as in all other seismograph prospecting systems so as to provide for recording of the instant of firing the shot.

invention has been described in refere'nce to reflection seismograph shooting. it ,is

' the shot point.

magnet is in-circuit through leads I and I" with a battery I" and swifoh arm III. A control switch I" is connected in parallel with leads Ill and Ill so that by closing the switch the pawl I" will be actuated. Control current leads l 01 from the amplifier output (of. Fig. 4) are connected to coil '4 of the ratchet switch I8, 82, ll through a rectifier l.- A switch "I is'c'onnected across the amplifier input to allow the operator to actuate this ratchet switch. Closing this switch results in production of a current in leads III, of sumclent strength to operate the ratchet switch, due to thermal or contact potentials.

- In operation. switch I" is first closed for a moment to bring the rotary switch arms II, 30 and I" to their extreme'right-hand positions resting againststops 18. Switch "I is then also applicable to refraction shooting, wherein.

similar advantages accrue. In such utilisation the detectors are of course spaced farther from shown, the detectors are bina- In the embodiments arranged for series connection when in tion. Theycan also be oonnectedin p series-parallel, and each detector can be replaced if desired by a plural combination of detectors soarrangedastoactasone. Theinventionis useful with various known geometrical arrangements of detectors and is not limited to any particular number-of detectors. Radio tr m can be employed in lieu oiwireltron if desired.

closed and opened one or more times to move the switch arms II, II and III to the desired detector position; e. g. to connect detector II to the amplifier. Upon the firing of the shot the rest,

of the operations are performed automatically. Each direct arrival produces a current in leads I" which serves to move switch arms 3!, SI and Ill to the next succeeding contact points; via. the arms first move from points I8, I! and I, to points 82, II and Ill. The last arrival brings the arm I to contact point Isl, thereby releas- 1. In seismograph prospecting methods where- I in the output oia plurality of seismic detectors is applied to a single recording element. the improvement which comprises locating a plural group of electric signal producing seismic wave detectors at spaced points in the earth, generating a seismic wave impulse at a point spaced from the group of detectors so as to cause propagation of seismic waves undergoing reflection and reing pawl Ill from ratchet wheel .3 and allowing fraction inthe earth, maintaining connection between only one ofsaid detectors and the recording element during the arrival of the elorfirst refractedwave at said detector and, before reflected waves arrive-at the detectors, establishing electrical connection between a plurality of the detectors and the recording element; whereby the first refracted wave is detected individually by a detector and the reflected waves'a-re detected by a plurality of detectors.

2. A seismograph prospecting apparatus for receiving and recording seismic waves comprising a plurality of detectors arranged to detect seismic waves and to produce electrical signals corresponding thereto, amplifying and recording means for said signals, and electronic switching means between'the detectors and the recording means-comprising a vacuum tube amplifying circuit for each detector, the circuit for the detector nearest the shot point being normally adjusted to amplify the signals from that detector and the circuits for the remaining-detectors being normally adjusted so as to be insensitive,.electrically from the last detector circuit and supplying it to v the sensitivity regulating means in all the preceding circuits in such sense as to render all the circuits sensitive in time for receipt of later reilected arrivals at the several detectors.

detectors with the amplifying andrecording means: the improvement which comprises switching means in and controlling said circuits,

adapted in one position to connect one of the" detectors to the amplifying andreconding means and in another position to connect a plurality of the detectors to the amplifying and recording means, means adapted on actuation to shift the switching means from the first position to the second and means for actuating said means at a time subsequent to receipt of the first refracted wave at the detector nearest the generating means butbefore receipt of reflected waves at the detectors.

5. A seismograph prospecting apparatus comprising in combination a plural group of electrical signal producing seismic wave detectors arranged in the earth, signal amplifying and recording means at a point spaced from the group of detectors, a switch having a plurality of contact elements greater than the number of detectors and a contact member movable with respect to said elements; circuits connecting the detectors separately to a plurality of the contact elements, a circuit connecting the detectors as a group to a pair of the contact elements, and a ,circuitconnecting the contact member with the amplifying and recording means: electrically operable actuating means for the contact member, and control means for said electrically operable means, located adjacent the amplifying and re- 3. A seismograph prospecting apparatus for receiving and recording seismic waves comprising a plurality of detectors for detecting seismic waves and producing electrical signals corresponding thereto, amplifying and recording means for said signals, and electronic switching means between the detectors and the recording means: said electronic switching means comprising a vacuum tube amplifying circuit for, each detector, the first circuit normally being adjusted so as to be sensitive and the succeeding circuits normally adjusted so as to beinsensitive,atransformerforeach amplifier circuit having a primary winding recording means, for operating the switch in some positions thereof to connect the detectors successively and individually to the amplifying and recording means, and in one position thereof to connect all the detectors to the amplifying and recording means. I

6. A seismograph prospecting apparatus comprising a source of seismic waves in the earth, a plural group of'electrical signal producingselsmic wave detectors spaced at various distances detector circuit adapted to maintain the circuit normally sensitive, adjusting means in each of the succeeding detector circuits adapted to ren- 'der the circuit normally insensitive, control ceiving the output of said circuit and a-s'econdary winding connected'to the recording means,

' an additional winding associated with the primary of the first amplifier circuit and means for applying energy from said additional winding to the first amplifier circuit in such direction as to render it insensitive, a plurality of control windings, one coupled with each of the transformer primaries except the last amplifier transformer primary, means for applying energy from each of said control primaries to thes'ucceeding amplifier circuit in such direction as to'render said succeeding amplifier circuit momentarily sensitive, an additional control winding coupled with the last transformer primary and means for applying energy from said last control winding to the preceding amplifier circuits, in such direction as to render them sensitive again.

4. In a seismograph prospecting apparatus comprising means for, generating seismic waves which penetrate in the earth and undergo refiection and refraction, a plural group of electrical signal producing seismic wave detectors spaced from 'said generating means, amplifying and recording means, and circuits connecting the first refracted wave detected at at least one of= means responsive to signal energy 'and'adapted to render the first detector circuit insensitive shortly after receipt of thefirst seismic signal 'thereat, a plurality of energy-supplying control circuits, one connecting each detector circuit with the adjusting means. of the succeeding circuit and adapted to render the succeeding circuit sen- I sitive on receipt of signal ener y,- and a control circuit supplying energy from the last detector circuit to the preceding circuits and adapted to of time sufilclent forreceipt of the later arrivin waves.

I. A method of seismograph prospecting comprising the steps of generating a seismic wave in the earth to propagate waves in the earth which are refracted and reflected beneath the surfacerof the earth, detecting waves at a plurality of points spaced in the earth such distances from the location ofwave generation that the first wave to reach each of said points is a refracted wave and later waves reaching each of said points are reflected waves, recording the said points singly while preventing during such recording the recording oi waves received at adiacent points, and combining and recording reflected waves detected at said plurality of points.

8. The method of claim 7 wherein the first retracted wave detected at only one of said points is recorded.

9. The method of claim 7 whereinthe first iracted wave and later waves reaching the detectors are reflected waves, a recorder and electrical connections between-all the detectors and the recorder, consists in maintaining operative electrical connection between only one of said detectors and the recorder before and during receipt or the first refraction wave at said detector and maintaining operative electrical connection between a plurality of said detectors and the recorder before and during receipt oi reflected waves at said detectors.

ENGELHARUI' A. ECKHARDT. GARY MUFFLY. 

